The present invention relates to a device for indicating that the level of a dosable material in a container has fallen below a predetermined required level, and more especially to such a device which comprises an element which is in contact with the dosable material and which supplies an electrical signal to an indicating device when the level of the material falls below the required level.
Among other applications, the device is suitable for use in electro-photographic copying equipment comprising containers which are filled with toner powder for replenishing a developer mixture. As is known, a latent image on a recording medium is developed in such a manner that a developer mixture, consisting of toner particles and carrier particles, is brought into contact with the electrostatic charge image on the recording medium. During the process of mixing with the carrier particles, the toner particles receive a triboelectric charge of a certain polarity. The electric field emitted by the charge image attracts the toner particles and detaches them from the carrier particles. Since this causes the developer mixture to be depleted of toner particles, toner must be added continuously to the developer mixture in order to keep the total concentration of toner in the developer mixture essentially constant. For this purpose there is a requirement for a device for sensing the actual level of the toner in the supply container. This device must promptly indicate that the toner is spent or that the actual level of the toner within the container has dropped below a certain mark so that the operating personnel will refill the container with toner. In this manner, the concentration of toner in the developer mixture is kept constant, since only this will ensure that the copying equipment will work in such a manner that the developed images are of uniform quality.
German Auslegeschrift No. 2,053,141 and corresponding British Pat. No. 1,302,687 disclose a device for determining the drop in the supply of a developer powder of an electrophotographic developing unit in a tank. In this device, operation of a signalling device indicates that the supply of developer powder is exhausted. This device is provided with a shaft which is rotatably mounted above and parallel to the bottom of the tank and a rod which is attached to the shaft by means of elastic elements parallel to and at a radial distance from it. This rod rotates with the shaft and has a blunt side which impedes penetration of the rod into the supply of developer powder. The elastic elements consist of several helical springs, the axes of which run radially out from the shaft, as long as the movement of the rod is not impeded. The distance of the rod from the shaft is increased by the elastic elements, if the supply of developer powder decreases. As soon as the distance from the shaft has reached a certain value, the signalling device is actuated. The springs hold the rod at such a distance from the shaft that the rod comes into contact with two electrical contacts if there is no longer any powder over the contacts, which causes an open signalling circuit to be closed and a lamp in this circuit to be supplied with current. The springs are sufficiently elastic so that they will be deflected when the contacts in the tank are covered with developer powder, thus causing the rod to float on the supply of powder and to slide through between the shaft and the electrical contacts without touching the latter, as long as there is sufficient developing powder in the tank.
This device is mechanically complex and requires considerable space which, though generally available in the developing tank of an electro-photographic device, is not available in the container of the toner-replenishing device. The container usually has a diameter which is only slightly larger than the diameter of the dosing brush located in the container and extends upwardly beyond the dosing brush. For this reason, the known device could not be accommodated in such a toner-replenishing device due to lack of space.
German Offenlegungsschrift No. 2,261,253 discloses a filling level-measuring device in which a sensing device, in contact with the toner particles, is movably mounted in a container and is moved, when the toner particles are at a distance from the sensing device, from a first into a second position. As a result of this movement, a drop of the level of the toner particles in the supply container below a level predetermined for the optimum operation of the copying machine is indicated. Upon rotation of this device from the first into the second position, a switch of an electrical arrangement is actuated in order to produce an electrical signal which triggers an indicating device. The sensing device consists of a bar magnet which is mounted on a rotatably mounted axle inside the storage container, and of two further bar magnets which are attached to the inner walls of the supply container. In the filled condition of the supply container, the bar magnet joined to the axle is immersed in toner and its longitudinal axis runs at approximately a right angle with respect to the longitudinal axes of the two bar magnets which are in line with each other. As soon as the supply of toner in the supply container has dropped to such an extent that the bar magnet located on the axle is freed, this bar magnet, under the influence of the magnetic field of the other two bar magnets, performs a 90.degree. rotation and comes to rest in one plane with these two bar magnets.
Due to the use of at least three bar magnets, this known device is relatively expensive to produce. Furthermore, before refilling the supply container with toner, the bar magnet located on the axle must be returned manually into its original position, that is to say the first position, in order to ensure a fault-free indication of the actual level.
Both known devices have the common feature that they are space-consuming and expensive since they consist of several mechanical parts and do not utilize already existing elements of the dosing device.